hspice as ad
It is not necessary to define AS, AD … You can do transient simulation but the results will not be correct, because the simulator (by default) supposes AD=AS=…=0.
AD, AS, PD and PS are dependent to the method of layout, and cannot be in model file (because nobody can anticipate how you want to draw the layout of transistor). So in order to do proper simulation you have to define them. But by using the command “.param” in hspice, you can simplify calculating AD ,… for each transistor.